1. Field of the Invention
The invention relates to a material, having high strength and good workability, that has been formed by rapidly deforming a base metal structure, such as illustratively a low carbon steel alloy, in order to generate a high rate of change in the internal energy of the structure which depressed the transformation temperatures of the base metal and thereby induced an allotropic phase transformation to occur therein.
2. Description of the Prior Art
Materials that undergo allotropic transformations are extremely important commercially and have been for many centuries. One class of these materials and probably one of the oldest known to man and the most widely used of all is steel. Not only does steel impart strength and rigidity to a product, but steel can also be formed into any one of a myriad of different shapes. For that reason, steel finds use in a wide array of different applications and particularly as an essential component of many products.
The chemical composition of a piece of steel, along with its thermal and mechanical history, determines its mechanical properties. Basic iron, i.e. iron without any impurities, is quite soft. As a result, various elements, such as carbon, are often dissolved into iron to change its physical characteristics. Specifically, steel is made by first forming molten iron from iron ore, limestone and coke that has been heated in a blast furnace. This molten iron (steel) often contains excessively high levels of silicon, manganese, carbon and other elements which adversely affect the physical properties of the resulting alloy. Consequently, the molten iron is placed into a basic oxygen furnace or an open hearth furnace to refine the molten iron with oxygen in an effort to reduce levels of the impurities to acceptably low values. Thereafter, the molten iron is then tapped or poured into refractory-lined ladles during which time other alloying elements and various deoxidizing materials are added to the steel to fix its final chemical composition.
Now, at this point, the steel is cast into ingots or slabs, either using molds or continuous casting processes. With the chemical composition fixed, the characteristics of the resulting steel can be varied by subsequent thermal and mechanical processing.
One of the most important properties of steel alloys is their ability to undergo allotropic transformations, here the ability of steel to change from a body centered cubic (bcc) crystalline structure to a face centered cubic (fcc) and back to bcc structure. Such transformations occur, without changes in chemical composition, because in certain temperature ranges one particular arrangement of atoms (e.g. bcc) that comprise a crystalline lattice is more stable (i.e. has a lower free energy state) than another arrangement. Inasmuch as the structure of the steel will always assume that arrangement which under equilibrium conditions yields the lowest free energy for a given thermal treatment, such transformations frequently occur with changes in temperature.
Different crystalline arrangements produce different mechanical properties. As such, controlling the allotropic transformations during the manufacture of steel greatly dictates the physical properties of the resulting steel. A large number of methods and processes exist to provide this control; however, the most common process is heat treatment using conventional furnaces, typically gas fired or electric, and suitable means of cooling, such as water or oil quenching or water, oil or gas spray. Generally speaking, a piece of steel is heated to a temperature above the transformation temperature. For eutectoid steels, the transformation temperature is a single value.
For low carbon steels, transformations occur throughout a range of temperatures, depending upon the heating rate and elapsed heating time. With an extremely slow heating rate and a long elapsed time value, the temperature at which the transformation begins is called the "Ae1" temperature and the temperature at which the transformation (from bcc to fcc) is complete is called the "Ae3" temperature. The "e" denotes equilibrium values. During heating or cooling, the Ae1and Ae3 temperature values shift thereby producing a band of values: a continuous heating transformation (CHT) curve for heating and a continuous on-cooling curve (CCT) for cooling. For heating or cooling, these values are denoted by the corresponding letter "c"--for the French word "chauffage" for heating or "r"--for the French word "refroidissement" for cooling. Once the steel has reached the Ac3 temperature it has been completely transformed into a high temperature product, which is typically austenite (a solid solution of carbon in fcc iron). Thereafter, once the steel is cooled below the Ar1 temperature, it has transformed back to a low temperature product, typically a bcc structure. The particular low temperature product that results is governed by the particular cooling procedures used. For example, ferrite (a solid solution of carbon in bcc iron) and pearlite (alternate lamellae of ferrite and iron carbide, the latter often referred to as cementite), which often exist together as a low temperature product, is generally formed by slowly cooling austenite using either furnace cooling or air cooling. Martensite, which is another low temperature product, occurs when austenite is rapidly cooled on an uninterrupted basis typically using oil or water quenching. If austenite is cooled at a rate between that for martensite and pearlite, then bainite may form. Bainite is another low temperature product and is a mixture of ferrite and cementite. Each low temperature product has different mechanical properties. A pure martensitic structure is the hardest and most brittle microstructure that can be produced in steel, while a pure ferritic structure is the softest. Pearlitic structures are considerably softer and more ductile than a fully martensitic structure but slightly less so than a pure ferritic structure. Consequently, heating and cooling procedures, coupled with prior mechanical working of the steel, influence the microstructure of the steel and its resulting physical properties. When low carbon steels are heated just above the Ac3 temperature and then cooled to room temperature, a fine grain structure results. This is a basic grain refinement procedure and may be performed several times to produce very fine grain structures. For a given hardness, the finer grained materials have higher strengths.
For many years, the art has taught that upon heating the Ac1 and Ac3 temperatures for low carbon steels generally increase from their equilibrium values with increasing heating rates. Specifically, see, for example, Y. Lakhtin, Engineering Physical Metallurgy (c. 1965: Gordon and Breach, New York) which states on page 161:
"Upon continuous heating at various rates . . . pearlite is transformed into austenite . . . , not at a constant temperature but in a certain temperature interval . . . The higher the heating rate, the higher will be the temperature of the transformation." [emphasis added]. PA1 "The constitutional [phase] diagram shows the position of the critical points under conditions of extremely slow heating or cooling and does not indicate their position when any other rate is employed. It is found that, when rates different from those specified under the conditions of the diagram are employed, the critical points do not occur at the same temperature on heating or cooling. This lag in the attainment of equilibrium conditions is termed hysteresis, which implies a resistance of certain bodies to undergo a certain transformation when this transformation is due. Therefore, the Ac point occurs at a temperature somewhat higher than would be expected. Similarly, the Ar point is somewhat lower. This difference between the heating and cooling criticals varies with the rate of heating or cooling. In other words, the faster the heating the higher will be the Ac point, and the faster the cooling the lower will be the Ar point." [emphasis added].
Similar teachings appear in E. J. Teichert, Metallography and Heat-Treatment of Steel (Ferrous Metallurgy- Volume III) (c. 1944: McGraw-Hill Book Company, Inc.; New York) which states on page 137:
In additional, similar teachings also appear on page 28.2 of the desk edition of The Metals Handbook (c. 1985, American Society of Metals; Metals Park, Oh.), on page 189 of C. Keyser, Basic Engineering Metallurgy-Theories, Principles and Applications (c. 1959: Prentice-Hall, Inc.; Englewood Cliffs, N.J.) and on pages 80-81 of L. Guillet et al, An Introduction to the Study of Metallography and Macrography (c. 1922: McGraw-Hill Book Company, Inc.; New York). Therefore, these teachings indicate that increasingly higher temperatures must be used to obtain transformations in increasingly shorter periods of time. This characteristic is typically found in diffusion controlled processes.
Now, having realized the importance transformations play in steel, it is now useful to discuss the typical manner in which a useable steel product, such as strip, is fabricated from ingot or slab (collectively referred to as ingots) and where transformations enter into the fabrication process.
Ingots are successively rolled to obtain thin strip stock. Each pass through a rolling mill reduces the thickness of the ingot and expands its length. To obtain large reductions in thickness, the ingot is first reduced in a roughing mill and then hot rolled through a hot strip mill. Hot rolling is performed at temperatures above the Ac1 and generally above the Ac3 temperatures. The typical hot rolling temperatures of between 850-1100 degrees Celsius (C.), steel has a relatively low flow stress and requires considerably less mechanical energy than in cold rolling to obtain a large reduction in thickness. In fact, very large reductions in thickness, on the order of an inch or more, are only possible during each pass through a roughing stand. At these temperatures, the steel exists as pure austenite. Hot rolled products generally exist in thicknesses of 0.06 inch (0.15 centimeters) or greater. The strength of hot rolled steel is somewhat higher than that of an annealed cold rolled steel; however, the formability of hot rolled steel is somewhat lower than an annealed cold rolled steel. Once hot rolling is complete, the steel strip is cooled at a controlled rate, typically using a water spray, to transform the austenite into a ductile low temperature product, such as ferrite and pearlite, prior to cold working and thereby prevent the steel from fracturing.
In general metallurgical practice, recrystallization is considered to be the result of heat treating steels below the Ac1 temperature. Any heat treatment above the Ac1 temperature may result in partially or fully transformed structures.
Where thicknesses less than 0.06 inches (approximately 0.015 centimeters), better surface finishes and/or improved" formability over that produced by a hot strip mill is required, the strip stock is further processed by cold rolling. Here, cold rolling generically refers to the process of passing unheated metal through rolls for the purpose of reducing its thickness. Now, to prevent the steel from fracturing, once hot rolling is complete, the steel strip is cooled at a slow controlled rate, typically using a water spray, to transform the austenite into a ductile low temperature product, such as ferrite and pearlite, prior to cold rolling. Cold rolling provides a product having a better surface finish and more precisely controlled dimensions than that which is possible through a hot mill. A typical five stand cold rolling mill may reduce the thickness of incoming strip by 75-90% with each stand generally being responsible for no more than a 40% reduction in thickness. During the rolling process, the temperature of the rolls rises due to plastic deformation of the material in the strip situated in the roll gap and frictional energy generated at each roll/strip contact. Because some of this energy remains in the strip, the temperature of the strip rises. In particular, strip is frequently at room temperature when it enters a cold rolling mill. After each rolling operation, the temperature of the strip as it exits from each stand is considerably higher than room temperature. For example, the temperature of the strip may reach 180 degrees C. as the strip exits the fourth stand in a five stand cold rolling mill. Inasmuch as the last stand (e.g. fifth stand in a five stand mill) is used to provide surface and leveling control of the strip, this stand imparts only a small reduction to the strip, typically ranging from a few percent to as much as 20% of the entering thickness. As such, the temperature of the strip as it exits from the fifth stand is often lower than that associated with the fourth stand but nonetheless considerably higher than room temperature. Throughout the cold rolling mill, the temperature of the strip is maintained, through use of suitable cooling sprays directed at both the strip and the rolls, well below temperatures at which the material in the strip would either transform or recrystallize.
As noted, cold rolling occurs below the recrystallization temperature, which is the temperature at which stressed, plastically deformed grains begin to recrystallize into new stress-free grains. Hence, equiaxed grains present in cooled hot rolled products are mechanically deformed into elongated (or banded) grains by cold rolling and remain in that state until subsequent heat treatment occurs. This deformation causes several effects, some of which are adverse.
First, cold rolling substantially distorts the crystalline structure of the steel strip and consequently substantially increases the density of dislocations present therein. This, in turn, increases the internal stresses occurring within the steel strip. Hence, the yield strength of a plain low carbon steel strip rises significantly, to on the average approximately 95,000 psi, while the ductility of the strip decreases significantly. Inasmuch as the amount of deformation a material will withstand before fracturing depends upon its ductility, a severely cold worked material may only accept a small amount of deformation before it fractures. However, to additionally deform the steel by further cold working, the ductility of the steel strip must be sufficiently high to prevent fracturing. Therefore, to obtain further large reductions in thickness by cold rolling, the steel strip may have to undergo one or more heat treatments to restore its ductility prior to subsequent cold rolling or fabrication. Such treatments reduce hardness and strength of the strip but advantageously increase its ductility. Moreover, the final strip produced by a cold mill is generally excessively hard and brittle for most application. To restore its ductility this final strip stock is annealed, i.e. heated in an annealing furnace into the austenitizing temperature range and then slowly cooled from this range to room temperature. This causes the elongated stressed ferrite and pearlite grains to first transform to austenite and then during slow cooling transform back into equiaxed stress-free ferrite and pearlite grains thereby relieving the internal stress within the strip. Alternatively, the strip could be heated to a temperature just below the Ac1 temperature, then held for an appropriate amount of time in order to allow the strip to recrystallize into stress-free grains and finally slow cooled. The resulting strip, having a yield strength on the order of approximately 30,000 to 50,000 psi depending upon the carbon content, is now capable of undergoing further significant cold reductions without fracturing. Annealing is typically done in a batch process using a slow heat-up, long soak and slow cooling cycle to ensure maximum formability. Annealing temperatures typically range between 730-950 degrees C. The entire batch annealing process may consume five to six days. To ensure that the annealing process does not cause a bottleneck to the entire steel mill, a number of separate annealing furnaces are operated at once but in staggered stages of annealing. Some furnaces are typically being loaded, while others are heating, others are cooling and the remainder are being unloaded. Unfortunately, such a staggered annealing process requires large amounts of capital to install and operate and consumes substantial amounts of space. Alternatively, continuous annealing lines, as discussed below, may be employed to reduce the total annealing time to less than one hour. Once the strip has been annealed, it may need to undergo a "skin" pass through a temper mill which imparts the desired flatness, metallurgical properties and surface finish to the strip stock. A skin pass typically involves imparting a very small amount of deformation, typically less than a few percent, to the finished strip and produces proportionate elongation of the strip.
Second, cold worked steel is directional. The elongated non-equiaxed grains produced by cold working impart different mechanical and electrical properties to the strip in directions parallel to and transverse to the direction in which the strip was rolled. For example, a cold worked unannealed strip is substantially more formable along a direction transverse to the rolling direction, i.e. perpendicular to the major axis of the grains, than along a direction parallel to the rolling direction. Both recrystallization and heat treatment through the transformation region eliminate all or some of the directional properties. For complete recrystallization to occur and thereby remove all effects of directionality, an annealing type heat treatment must be used to allow the steel to recrystallize into an equiaxed grain structure. Alternatively, the material may be completely transformed to austenite and then slow cooled to room temperature to produce a completely transformed structure, i.e. a completely annealed equiaxed structure.
As noted above, continuous strip annealing lines have been developed which anneal the strip in less than one hour. In such a line, the steel strip is passed at mill speed through separate heating and cooling zones, where the strip is heated, held at temperature and cooled or quenched. This process may be done at different rates which may change during any part of the process. Moreover, such a line is often designed to heat treat the strip several times as it passes through the line. In order to quickly elevate the temperature of the material into the austenite region, very high temperatures are used. Although this produces an end product of uniform structure, it does so at considerable cost. Specifically, strip annealing mills are expensive, typically over $200 Million, to acquire and install. Second, high temperature heat treatments cause an oxide layer ("scale") to build up on each surface of the strip. The amount of oxide increases with time at temperature. Therefore, additional machinery is needed to remove this scale from each surface. Although most continuous annealing lines include surface cleaning equipment, this equipment adds to the cost of the line. Alternatively, the scale can be eliminated by shrouding the steel, as it travels through the continuous annealing line, with an inert or reducing atmosphere. However, the cost of the equipment needed to do so adds expense to the continuous annealing line, both in terms of initial cost and subsequently incurred operating costs.
Therefore, in view of this manufacturing process, cold rolled low carbon steel alloys present a tradeoff: non-annealed cold rolled products possess relatively high values of yield strength and hardness and a correspondingly low degree of formability, while annealed products provide a high degree of formability and relatively low values of yield strength and hardness--typically less than one half that of the non-annealed cold rolled products. Although, low carbon steel alloys comprise the least expensive of all commercially available steel alloys and, for that reason, are widely utilized, a single piece of a low carbon steel alloy does not provide both high strength and high formability. As a result, a user decides which of these two characteristics, high strength or formability, is more important in any given application and chooses a material accordingly. However, in those applications, where a formability-strength tradeoff can not be tolerated, i.e. where a steel must possess both high strength and good formability, a high strength low alloy (HSLA) steel or other types of steels are frequently used instead of low carbon steel. Unfortunately, such steels are more difficult to produce and hence considerably more expensive than low carbon steels. In addition, these steels are often harder to weld and form than low carbon steels.
Furthermore, the production of "black plate" as it is used in the making of tin plate provides another example where present processes taught in the art are inadequate to provide suitable material for an end use. In particular, U.S. Pat. Nos. 2,393,363 (issued to J. D. Gold et al on Jan. 22, 1946 --hereinafter referred to as the '363 Gold et al patent) and 3,323,953 (issued June 6, 1967 to A. Lesney--hereinafter referred to as the '953 Lesney patent) disclose methods which are aimed at obtaining a material, such as a strip, having a strong core and a soft surface. The '363 Gold et al patent discloses use of conventional heat treatments to obtain recrystallization of the surface but no recrystallization of the core. Specifically, a suitable material is surface heated to a relatively high value, here 1500 degrees F. (approximately 816 degrees C.), sufficient to cause recrystallization of the surface. Once the material has recrystallized to a desired depth, heating is stopped and the material is then appropriately cooled to remove any excess heat and, by doing so, inhibit any further recrystallization. The '953 Lesney patent discloses the use of a special material where the surface region contains material that is more susceptible to recrystallization than the material situated in the core. Specifically, the special material, here rimmed steel with a maximum manganese content of less than 0.15%, is annealed in strip form at a relatively high temperature, here 800-1150 degrees F. (approximately 427-621 degrees C.), for a time sufficient to substantially recrystallize the surfaces of the strip, but insufficient to recrystallize the core of the strip.
Prior art processes based upon surface recrystallization, such as those disclosed in the '363 Gold et al and '953 Lesney patents, possess several drawbacks which significantly limit their commercial use. First, these processes depend upon imparting a controlled amount of heat at a desired depth in a material being processed. The amount of heat that a material absorbs varies with many factors, such as for example conduction by surrounding air and reflectivity of the surface of the material. Unfortunately, these factors may vary for different materials and even for different pieces of the same material thereby complicating the control of the heating process. Moreover, since recrystallization is a diffusion controlled process, it is time dependent. Frequently, a fairly long interval of time typically lasting several seconds, if not minutes, is required for a material or even a portion of it to recrystallize. As such, heating a material to impart a controlled amount of heat to a certain desired depth from its surface throughout a particular period of time is extremely difficult to accurately accomplish on a repetitive basis with different pieces of the same or different material.
Consequently, a need exists in the art for materials, formed from illustratively inexpensive low carbon steel alloys, that provide both higher strength and higher formability than various materials currently available.